1. Field of the Invention
The present invention relates to a drive and position control of a diaphragm. In particular, the present invention relates to a technique in which power consumption can be lowered in consideration of responsiveness of moving image exposure.
2. Description of the Related Art
Currently, in what is referred to as a “single-lens reflex type” digital camera of which the interchangeable lens is removable, a product that has a live view function that displays a preview image prior to photographing a still picture on a TFT monitor disposed on the rear face of a casing or is capable of photographing a moving picture is known. Conventionally, there has been required high-speed responsiveness and high positional accuracy for focus driving, diaphragm driving, and the like of an interchangeable lens, since the primary aim was still picture photographing. For this reason, a stepping motor excellent in driving characteristics and controllability tends to be employed as an actuator for driving the diaphragm. The drive methods of a stepping motor includes 1-phase excitation drive, 2-phase excitation drive, 1-2-phase excitation drive, and the like, which may be provided for separate use in accordance with applications.
In order to ensure that a stepping motor is employed as the actuator for the diaphragm to stop the diaphragm to hold it at that particular position, it is required that a holding current continuously flows through a coil of the excitation phase at the stop position. A holding current itself may be smaller than the current used during driving. However, this is a direct current, and, if this state persists for a long time particularly when a battery is employed as the power supply, power may be consumed and heat generated despite the fact that the diaphragm is not being driven. This particularly creates problems when a battery is employed for a power supply.
In the 1-phase excitation drive, even if a current supplied to a stepping motor is shut off in a stop state, the magnetic poles of the rotor and the stator face to each other, and the stop position is held by the magnetic force of the rotor. However, the rotation stepping angle per step is large, which is not suitable when highly accurate positioning, such as a diaphragm, is required.
In the 2-phase excitation drive, the rotor stops at a position intermediate between two phases. Hence, in order to stably hold the stop position, a minimum holding current must continuously flow through the coil in the stop state. As a result, power consumption and heating occur.
In the 1-2-phase excitation drive that combines the 1-phase excitation drive and the 2-phase excitation drive, the rotation stepping angle per step becomes as half that of the driving methods, and thereby a fine control can be carried out. In the 1-phase stop position, the stop position can be held even if a current is shut off, whereas in the 2-phase stop position, a holding current is required. Even if the holding current is decreased to a limit value at which the stop position of the rotor can be held, the holding current cannot be ignored when the use of the battery power supply is considered.
As described above, realizing a satisfactory drive control by using any of the drive methods is difficult. In particular, when exposure control is performed by a diaphragm during moving picture photographing or the like, accurate controllability of the diaphragm and holding it at the stop position are required.
In order to solve the aforementioned problems, a method for driving a focus lens, which is disclosed in Japanese Patent Laid-Open No. 7-77648, is known. In this method, even when the stop position needs not to be strictly controlled by adjusting the depth of field or the like and the stepping motor is at a 2-phase stop position, the stepping motor is rotated by 1-step within the range of the depth to thereby stop at a 1-phase position. When the subject is within the depth of field, the rotor is rotated by 1-step to a position of a 1-phase excitation phase and then the electricity is shut off, whereby a holding current becomes zero. When the subject is outside the depth of field, the current limitation is performed.
However, when the method disclosed in Japanese Patent Laid-Open No. 7-77648 is applied to a method for controlling a diaphragm under the exposure control of a moving image, there are problems with controllability and power consumption. For example, when an imaging lens unit that provides poor controllability of the diaphragm is mounted on a camera body, a problem such as flickering or the like occurs by the operation by which the rotor is moved to a position of a 1-phase excitation phase. Also, when long term photographing such as moving picture photographing or the like is expected, power consumption becomes large even if current limitation is performed. Therefore it is desired that the diaphragm be held when the holding current is zero